People speaking English as a second language find each other just as
intelligible as they do native English speakers, US linguists have found. The
effect works regardless of the speaker's mother tongue.

It isn't hard to see why a Korean, say, might find another Korean's
English easier to follow than an English person's. The two share a phonetic
vocabulary lacking some of the vocal effects that render the language alien in
a native's mouth. A foreign accent hinders a native but helps a fellow
non-native.

But what about speakers with different first languages? One might
suspect that only some languages, like Korean and Chinese, or Spanish and
Italian, share sounds that help their mutual intelligibility. But that doesn't
seem to be so.

Instead, there may be features of the target language that all
non-natives omit, suggest Tessa Bent and Ann Bradlow from Northwestern
University in Illinois1. American English speakers often
fail to sound consonants at the ends of words clearly, for example, making it
hard for non-natives to tell one word from another.

Received
pronunciation

It is often claimed that two non-natives communicate more easily in a
second language than either would with someone born speaking that language.
That's to say, Romanians might find Romanian-accented English more intelligible
than native English.

But there's been little hard evidence to support this. What's more,
little is known about what happens when non-native talkers have different first
languages, says Bent and Bradlow.

The duo assembled a veritable Babel from students at an American summer
school for learning English. Their subjects included Chinese, Koreans,
Bengalis, Hindi speakers, Japanese, Romanians, Slovakians, Spaniards and Thais,
as well as American English speakers.

Participants took turns speaking and listening. They were recorded
saying simple English phrases such as 'The dog came back', and assessed for
their intelligibility.

As long as the proficiency of the speakers was not too low, non-natives
found each other at least as intelligible as native English speakers,
regardless of whether they shared a first language, Bent and Bradlow found.
Natives found each other more intelligible than non-natives, just as one would
suspect.